1. Field of the Invention
The present invention relates to a super wide-angle lens system and in particular to a fast super wide-angle lens system having a wide angle of view suitable for a single-lens-reflex camera using silver-halide film or a solid-state imaging device. The present invention relates also to an image-capturing device equipped with the super wide-angle lens system.
2. Related Background Art
Up to now, not many proposals regarding a super wide-angle lens system having an angle of view of 2ω=100° or more with ordinary projection method (y=f·tan θ) have been made. Furthermore, proposals of a fast super wide-angle lens system with the f-number of about f/2.8 have been extremely rare. Super wide-angle lens systems like this have been proposed by the applicant of the present invention in Japanese Patent Application Laid-Open Nos. 5-34592 and 2001-159732.
However, super wide-angle lens systems disclosed by the above-mentioned patent documents correspond to an imaging area of a silver-halide film camera and not that of a digital camera. Accordingly, in order to construct a super wide-angle lens system with an angle of view of 2ω=100° or more in an imaging area of a digital camera, since the focal length becomes short, it becomes difficult to secure back focal length.
Moreover, in a super wide-angle lens systems proposed in Japanese Patent Application Laid-Open No. 5-34592, the ratios of the back focal length to the focal length are from 2.73 to 2.75. Accordingly, when a shorter focal length is to obtain with the super wide-angle lens system, it may produce a problem to interfere with a quick return mirror of an SLR camera.